| In recent years, with the rapid development of microelectronics and electronic packaging technology, the cooling efficiency, insulation and flame retardant properties of logic circuits and electronic components become increasingly demanding. Phenolic resin is inexpensive, has simple production process, good chemical resistance and high mechanical strength, it has been widely used in friction materials, ablation materials, coatings and molds. However, the phenolic hydroxyl and methylene in traditional PF structure have great activity, so the resin is easily oxidized. The limited heat resistance and brittleness all hinder PF from further development. Therefore PF modification research is imperative.Current studies on thermally conductive composites mainly focused on how to facilitate dispersion and reduce interfacial thermal resistance by surface treatment, filler with different shapes synergize to form the three-dimensional thermal network, as well as different sizes of thermally conductive filler compounded to increase the amount of filling to further improve the thermal conductivity of the composite. The study for the degradation of processing and mechanical properties which was caused by high loading is lacking.To solve these problems, this thesis is scheduled to improve the thermal conductivity and toughness of the high loading polymer composites. We use high-performance fibers as the toughening component, through system design, composite processing-microstructure-property relationships study, to explore the effective way of toughening highly filled thermally conductive composites, and ultimately to prepare the highly filled composites with both high thermal conductivity and excellent mechanical properties, and provide new technology for the fast-growing society to meet the huge demand for new materials. The main content is as follows:(1) With thermosetting phenolic resin as matrix, we prepared PF/Al2O3 binary composites by high pressure molding, and adopt single factor experiment to study the effect of the alumina particle size, different content, different size compounding and modification on the thermal conductivity of the composites. The larger size, the easier to form conduction path, and obtained higher thermal conductivity. Keep the same content, the thermal conductivity filled with 40μm is higher than 10μm and 5μm;. Before filling 60 wt%, the thermal conductivity of the composites increases rapidly, however continue to increase the filler amount, the trend becomes slow, and the system is too viscous to process difficultly; fixing total filler content with 60 wt%,5 μm was added in various proportions, however the thermal conductivity decreased; The modification with KH550 also has little effect on thermal conductivity. In summary, we choose 40μm spherical Al2O3 and the content of 60 wt%, the thermal conductivity can be 0.78 W·m-1·K-1,3.9 times higher than that of pure PF (0.20 W·m-1·K-1).(2) High filling will inevitably lead to the deterioration of mechanical properties, together with the intrinsic brittleness of phenolic resin, the notched impact strength of pure PF is only 0.61 kJ·m-2, cannot effectively absorb the energy of impact damage. Therefore, fixing with 40μm and 60 wt%, we attempt to add carbon fiber and flexible Kevlar fiber as the toughening component and prepared PF/Al2O3/fiber ternary composites. Experimental data show that the carbon fiber did not obtain very satisfied toughening effect. With 5 wt% carbon fiber, the notched impact strength of composites increased to 3.56 kJ·m-2, and then continue to increase filler content, the strength show a clear downward trend, may be due to the aspect ratio of fiber is destroyed during the preparation and cannot form the conductive network. However the composites obtain a better toughening effect after adding Kevlar fiber. With the increase of fiber content, the notched impact strength increased gradually, with 8 wt% Kevlar, the notched impact strength is 6.8 kJ·m-2,11.3 times higher than that of pure PF (0.6 kJ·m-2), at the same time, did not damage the bending properties and thermal conductivity, the thermal conductivity increased to 0.85 W·m-1·K-1, is 4.25 times than that of pure PF (0.20 W·m-1·K-1). We finally prepared a highly filled PF/Al2O3/Kevlar (32/60/8) composites with both high thermal conductivity and excellent mechanical properties. |
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